1. Field of the Invention
The present invention relates to a geometrical shape of a spiral body built-in to a scroll type compressor suitable for use in an automobile air-conditioner.
2. Description of the Related Arts
It is preferable to thin a wall thickness of a spiral body (hereinafter referred to as "scroll") to reduce the weight of a scroll type compressor, but the scroll is subjected to a severe counteraction of a varying compression of a medium gas. This is particularly true of the start area of the scroll, as this area is exposed to a maximum pressure, and accordingly, at least this area of a scroll should have a wall thickness sufficient to withstand such a pressure and avoid damage due to wear. In the conventional scroll type compressor, an involute curve is used as a profile of outer and inner walls of both the movable and stationary scrolls, and therefore, the wall thickness is uniform over all of the wall length. Accordingly, if the start area of the scroll has a sufficient wall thickness, it continues to the end area thereof, and thus a thinning of the scroll wall becomes impossible.
A solution is proposed in Japanese Unexamined Patent Publication (Kokai) No. 60-98186, in which a wall thickness of a movable scroll is gradually reduced toward an end area thereof, and a wall thickness of a stationary scroll is increased correspondingly. Profiles of both the outer and inner walls are involute curves, and a basic circle of the outer wall curve has a smaller diameter than that of a basic circle of the inner wall curve. The use of the basic circles, each having a different diameter, enables the wall thickness of the movable scroll to be made thinner toward the end area thereof. The reduction of the wall thickness of the movable scroll is compensated by the increase of that of the stationary scroll, so that a smooth contact between both scrolls can be ensured during the orbital motion of the movable scroll.
According to the above-mentioned proposal, nevertheless the weight of the movable scroll is reduced when enhancing the mechanical strength of the start area thereof, the weight of the stationary scroll is conversely increased, and therefore, the total weight of the compressor cannot be reduced. Further, as the profiles of the outer and inner wall are still involute curves, a reduction of a diameter of the scroll cannot be attained, which is essential to the compactness of this type of compressor.